Field of the Invention
The present invention relates to a scintillator panel having excellent luminance, sharpness, and formability, and a method for manufacturing the same.
Description of the Related Art
Conventionally, a radiation image such as an X-ray image has been widely used for diagnosis of a disease at a medical site. Particularly, an intensifying paper-film type radiation image has enhanced sensitivity and image quality thereof in the long history. As a result, the intensifying paper-film type radiation image is still now used widely at a medical site in the world as an imaging system having both high reliability and excellent cost performance. However, this image information is so-called analog image information, and cannot perform image processing freely or cannot perform electrical transmission instantaneously unlike digital image information which is developing now.
As one of digital technologies on an X-ray image, computed radiography (CR) is now accepted at a medical site. However, an X-ray image obtained by CR has insufficient sharpness and insufficient spatial resolution compared to an image obtained by a screen film system such as a silver salt photography method. The image level of CR has not reached that of the screen film system. Therefore, as a new digital X-ray image technology, for example, a flat panel X-ray detector (FPD) using a thin film transistor (TFT) has been developed.
In order to convert an X-ray into visible light, the above FPD principally uses a scintillator panel including a scintillator layer formed with an X-ray phosphor which converts an irradiation X-ray into visible light to emit light. However, in X-ray imaging using an X-ray source having a low dose, in order to increase a ratio (SN ratio) between a signal and a noise detected by the scintillator panel, it is necessary to use a scintillator panel having a high luminous efficiency (conversion ratio of an X-ray into visible light). In general, the luminous efficiency of a scintillator panel depends on the thickness of a scintillator layer and an X-ray absorption coefficient of a phosphor. The thicker the scintillator layer is, the more easily the light emitted by X-ray irradiation in the scintillator layer is scattered. An excessively thick scintillator layer deteriorates sharpness of an X-ray image obtained via the scintillator panel disadvantageously. Therefore, when sharpness required for an image is determined, the film thickness is determined automatically. Therefore, a scintillator plate which has an excellent luminous efficiency, that is, has both excellent luminance and excellent sharpness (MTF), and can form a high image quality, has been desired.
JP 2007-292583 A discloses a scintillator plate using at least one kind selected from gadolinium oxide containing an activation material and gadolinium oxysulfide containing an activation material as a phosphor. Each of the gadolinium oxide and the gadolinium oxysulfide is a mixture of particles having different average particle diameters. However, the scintillator plate described in JP 2007-292583 A requires further improvement in emission luminance and sharpness.
JP 5340444 B1 discloses a radiation image detector including a wavelength conversion layer having a first phosphor layer and a second phosphor layer in such an order that the spatial filling ratio of the phosphor particles increases on aside of the detector in order to improve sharpness. The first phosphor layer and the second phosphor layer each have phosphor particles dispersed in a binder. The average particle diameter of the phosphor particles in the second phosphor layer is smaller than that of the phosphor particles in the first phosphor layer. JP 2013-217913 A discloses, a radiation image detector including a wavelength-converting layer having a monolayer phosphor layer in which first phosphor particles having a first average particle diameter and second phosphor particles having a second average particle diameter are mixed in a binder in order to improve sharpness. The second average particle diameter is smaller than that of the first average particle diameter. The weight of the phosphor particle is gradually decreased as the distance from the solid detector is increased.
However, the technology disclosed in JP 5340444 B1 or JP 2013-217913 A distributes phosphor particles in a phosphor layer nonuniformly in a direction perpendicular to a surface of a support, requires a very high ability for controlling a process in order to control a dispersion state of the phosphor with a fixed order, and is not necessarily suitable for industrial mass production. Because of the nonuniform distribution of the phosphor particles in the phosphor layer, it cannot be said that light emitted by a phosphor particle existing on the opposite side to a sensor panel can be received efficiently.
In such a situation, appearance of a new scintillator panel which has high luminance and sharpness and does not require complicated management of a process is desired strongly.